Why is Japanese cedar pollen allergy such as problem?

I knew it was a man-made problem, but didn’t know the details until I read The Economist this week. After WWII the sugi trees were planted to provide material to rebuild houses, but after import tariffs fell it became too unprofitable to even be worth cutting the trees down. It’s not just the sheer number of trees that is responsible, though – as they grow higher, each tree emits more and more pollen every year.